Method for centering a workpiece on the cylindrical axis of a bore

ABSTRACT

A method and system for precisely centering a workpiece within a cylindrical bore such that the center of the workpiece is coaxial with the central axis of the bore. An arbor is used within the cylindrical bore to sense and calculate the location of the central axis. The position of the workpiece is also sensed so that it can be aligned with the central axis. A computer system is used to perform the calculations and provide an indication of alignment of the workpiece and the central axis. The position of the central axis and the workpiece are found via a location determining system that may incorporate electromechanical and optical position sensing systems. For instance, a plurality of light transmitters and light receivers may be used to generate and transmit beams of light used in determining positions. The method has particular application to free piston machines, and particularly free piston coolers.

TECHNICAL FIELD

The present invention relates to the field of equipment positioningsystems and, more particularly, to a method and system for preciselypositioning a workpiece on the central axis of a cylindrical bore.

BACKGROUND OF THE INVENTION

Various industries that manufacture and assemble machinery having closetolerance, reciprocating parts have recognized the importance of havingprecise alignment among the reciprocating parts and the stationarycomponents. The importance of such alignment may be explained using thewidely applicable example of a piston reciprocating in a cylindricalbore. The simplest example is a piston that is free to reciprocatewithin the cylindrical bore with its path of reciprocation in the borenot restricted by anything except the bore. The movement of such apiston is guided by the surrounding walls of the cylindrical bore sothere is no need for alignment.

However, if such a piston also has an axial hole through the center andslides on an axially aligned rod passing through the hole, the path ofreciprocation is determined by both the walls of the cylindrical boreand the rod. Therefore, if the rod upon which the piston slides issufficiently out of alignment with the central axis of the cylindricalbore, then at some point during the stroke of the piston where the rodis displaced radially from the central axis, the clearance, which shouldbe a constant throughout the length of the stroke, will becomenonexistent and excessive contact will occur. Such contact is often verydamaging to the equipment and extremely difficult to repair, if notirreparable. Additionally, such contact will often wear down the variouscomponents thereby reducing the efficiency of the machine or causing itto malfunction.

In the past, centering of such components was performed manually. Humanerrors, compounded with errors from manual measurement equipment, didnot provide the level of precision that is required by today'stolerances. Additionally, such manual determinations are difficult andtime consuming for even the most skilled machinist. Further, often thealignment needs to be performed to a greater accuracy than can beachieved by a purely mechanical device.

More recently, semi-automated centering devices have been developed butthey generally include a multitude of gages, meters, and indicators thatmust be carefully attached to the bore and are often extremelysensitive. Further, it is often necessary for such gages, meters, andindicators to extend into the bore to perform their functions, makingthem difficult to use, adjust, and read. Advanced optical alignmentsystems do exist, however they often require precision optics to producepredetermined patterns of light and are often extremely sensitive andcost prohibitive.

Precise centering of reciprocating components within a cylindrical boreis a need often encountered in the field of free piston machines,particularly Stirling devices such as Stirling engines and coolers. Theefficiency of such devices is dependent upon exacting tolerances betweenthe stationary and reciprocating components. In fact, the radialclearance between such components is often 12 to 13 microns.

Free piston coolers operating with such close tolerances generallyincorporate linear gas bearings as well as specially designed surfacecoatings, such as fluoropolymers, on the reciprocating components. Theradial loading on the linear gas bearings is minimized by attempting tomaintain a consistent clearance throughout the stroke of thereciprocating components. In order to maintain a consistent clearancethroughout the stroke of a free piston machine, the rod that passesthrough a hole in the piston along which the piston slides, such as thedisplacer rod, must be perfectly centered within the bore. When thedisplacer rod is not perfectly centered, the piston sidewalls approachthe bore walls at some point during the stroke thereby getting closerthan the desired clearance and increasing the radial bearing load. Onlya slight misalignment of the displacer rod may result in contact betweenthe piston and the bore. Therefore, misalignment of the displacer rodresults in reduced bearing life as well as undue wear on the variouscomponents of a free piston machine.

Accordingly, there is a need for a precise centering method and systemthat is economical, portable, and easy for an unskilled operator to use.While some of the prior art devices attempted to improve the state ofthe art of centering systems, none have achieved the beneficialattributes of the present invention. With these capabilities taken intoconsideration, the instant invention addresses many of the shortcomingsof the prior art and offers significant benefits heretofore unavailable.

SUMMARY OF INVENTION

This invention recognizes that numerous industries require the abilityto accurately center a workpiece within a cylindrical bore such that thecenter of the workpiece is coaxial with the central axis of thecylindrical bore. This is particularly true in applications wherein theworkpiece reciprocates within the cylindrical bore.

The present invention is directed to precisely centering a workpiecewithin a cylindrical bore such that the center of the workpiece issubstantially coaxial with the central axis of the bore. The methodfirst senses and calculates the location of the central axis of the boreand then senses and calculates the position of the workpiece so that itcan be aligned with the previously determined central axis. An arborthat precisely fits within the cylindrical bore is used in determiningthe location of the central axis. The arbor has a reference pinextending from one end that is centered on the arbor. It is the locationof this reference pin that is sensed to determine the location of thecentral axis.

Once the central axis is determined, the arbor is removed from thecylindrical bore and the workpiece is inserted into the bore. Thelocation of the workpiece is then sensed and the center of the workpieceis calculated. The center of the workpiece is compared to the locationof the central axis and the location of the workpiece is adjusted untilthe center of the workpiece and the central axis are substantiallycoincident.

A computer system receives data representative of the location of thereference pin and the workpiece and calculates the center of each. Thesecenters are referred to as the bore axis target and the rod target. Thetargets may be visually displayed on a computer display or they maymerely be coordinates resident in the memory of the computer system.Regardless of the representation of the targets, they serve as anindication of the current position of the workpiece and the finaldesired position of the workpiece. An individual, or an automatedsystem, may then assess the location information and adjust the locationof the workpiece to obtain the desired location.

The position of the reference pin and the workpiece is found via alocation determining system which may incorporate numerous positionsensing technologies, including, but not limited to, electromechanicalsystems and optical systems. For example, a mechanical positiontransducer may be used to sense the locations and generate an electricalsignal representative of the position data that is transmitted to thecomputer system. Alternatively, a plurality of light transmitters andlight receivers may be used to generate and transmit beams of light fromeach of a plurality of light transmitters across the central axis tocooperating light receivers. The reference pin or the workpieceinterferes with the transmission of the beam of light from each lighttransmitter to the cooperating light receiver resulting in a shadowbeing cast upon the opposing light receiver. The location of theseshadows on the light receivers provides an indication of the location ofthe reference pin and workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Without limiting the scope of the present invention as claimed below andreferring now to the drawings and figures:

FIG. 1 is a partial cross section view, not to scale, of a body having acylinder in accordance with the present invention;

FIG. 2 is a partial cross section view, not to scale, of the body and anarbor of the present invention;

FIG. 3 is a top plan view, not to scale, of the body and arbor alongwith a schematic of the location determining system and the computersystem displaying the central axis of the cylinder in accordance withthe present invention;

FIG. 4 is a partial cross section view, not to scale, of the body with aworkpiece to be centered in accordance with the present invention;

FIG. 5 is a top plan view, not to scale, of the body and workpiece alongwith a schematic of the location determining system and the computersystem displaying the central axis of the cylinder and the position ofthe workpiece in accordance with the present invention;

FIG. 6 is a partial cross section view, not to scale, of a free pistoncryocooler showing the component being centered in accordance with thepresent invention;

FIG. 7A is a partial cross section view, not to scale, of the stationarycomponents of the free piston cooler of FIG. 6 in a partiallydisassembled condition;

FIG. 7B is a partial cross section view, not to scale, of thereciprocating components of the free piston cooler of FIG. 6 in apartially disassembled condition;

FIG. 8 is a partial cross section view, not to scale, of a portion ofthe free piston cooler and arbor of the present invention;

FIG. 9 is a top plan view, not to scale, of a portion of the free pistoncooler, arbor, and optical system of the present invention;

FIG. 10 is a partial cross section view, not to scale, of a portion ofthe free piston cooler and workpiece of the present invention;

FIG. 11 is a partial cross section view, not to scale, of a portion ofthe free piston cooler, workpiece, and optical system of the presentinvention, taken along section line 11—11 in FIG. 10;

FIG. 12 is an elevated perspective view, not to scale, of the freepiston cooler, arbor, and optical system of the present invention;

FIG. 13 is an elevated perspective view, not to scale, of the freepiston cooler, displacer, displacer rod, piston, displacer spring, andoptical system of the present invention; and

FIG. 14 is an elevated perspective view, not to scale, of the freepiston cooler with the displacer, displacer rod, piston, and displacerspring installed and ready for centering, and optical system of thepresent invention.

Also, in the various figures and drawings, the following referencesymbols and letters are used to identify the various elements describedherein below in connection with the several figures and illustrations:CX, LB, Md, Mp, and Q.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the drawingsis intended merely as a description of the presently preferredembodiments of the invention, and is not intended to represent the onlyform in which the present invention may be constructed or utilized. Thedescription sets forth the designs, functions, means, and methods ofimplementing the invention in connection with the illustratedembodiments. It is to be understood, however, that the same orequivalent functions and features may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention.

With reference generally now to FIG. 1 through FIG. 5 which illustratethe basic principles of the invention, in one of many preferableconfigurations, the method and system of the present invention aredirected to precisely centering a workpiece 200, having a rod 210, onthe central axis CX of a cylindrical bore 110 in a body 100. The methodincludes two distinct steps; first, sensing the location of the centralaxis CX of the bore 110 and, secondly, sensing the position of the rod210 and aligning the center of the rod 210 with the central axis CX.

In determining the location of the central axis CX of the bore 110, anarbor 300 is inserted into the proximal end 114 of the bore 110. Thediameter of the arbor 300 is slightly less than the diameter of the bore110 so that the cylindrical exterior surface 310 of the arbor 300matingly slides within the cylindrical bore 100, in contact with thebore interior surface 112. The arbor 300 has a symmetrical reference pin320 protruding from an end of the arbor 300. Precise machining of thecylindrical bore 100 and the arbor 300 ensure that the center of thereference pin 320 is coaxial with the central axis CX of the bore 110.The bore 110 has a proximal end 114, which is open during application ofthe present method, and a distal end 116, which may be closed.Additionally, the bore 110 may have a diameter 118 that is constant fromthe proximal end 114 to the distal end 116, or the bore 110 may have anumber of sections of different diameter, as illustrated in FIG. 1, FIG.2, and FIG. 4. As seen in FIG. 2, the method generally uses an arbor 300and a reference pin 320 configured such that when the arbor 300 isinserted into the bore 110, the reference pin 320 extends beyond theproximal end 114 of the bore 110 to simplify the sensing of the locationof the reference pin 320, however, this is not required. The referencepin 320 provides a reference surface on the arbor 300.

Next, the position of the reference pin 320 in a plane transverse to thecentral axis CX is sensed and pin position data is transmitted to acomputer system 400. The position of the reference pin 320 is found viaa location determining system 500. The location determining system 500may sense the location of the reference pin 320 in any number of waysincluding, but not limited to, electromechanically and optically.

For example, a mechanical position transducer may be used to sense thelocation of the reference pin 320 and generate an electrical signalrepresentative of the pin position data that is transmitted to thecomputer system 400. Such a mechanical position transducer is acontact-type transducer whereby sensors physically contact the referencepin 320 and the signal may be analog an AC or DC voltage or current(4–20 mA), or a digital data signal, for example.

Alternatively and preferably, the location determining system 500 may bean optical system. An optical location determining system 500 comprisesapparatus for generating and transmitting a beam of light from each of aplurality of light transmitters across the central axis CX tocooperating light receivers. The reference pin 320 or the rod 210, aswill be described later, interferes with the transmission of the beam oflight from each light transmitter to the cooperating light receiverresulting in a shadow being cast upon the light receiver. The locationof the shadow on the light receiver provides an indication of thelocation of the reference pin 320. Generally, a plurality of light beamsare transmitted from light transmitters to light receivers at apredetermined spacing on opposite sides of the cylindrical bore 110 tosense the location of the reference pin 320. In one particularembodiment, a pair of cooperating light transmitters and lightreceivers, that are orthogonal to one another, are utilized to generateand transmit two light beams across the central axis. One specificembodiment of an optical location determining system 500 will bediscussed in detail later herein with particular reference to opticallycentering the displacer rod of a free piston cooler. Further, any of anumber of various types of light transmitters and light receivers may beused in the present invention. In one embodiment, light transmittersincorporating light emitting diodes to generate the beams of light andlight receivers having charge coupled devices are used.

The computer system 400 receives the signals from the locationdetermining system 500 and then computes the center of the reference pin320 from the pin position data. The computer system 400 then representsthe center of the reference pin 320, in other words, the central axis CXof the bore 110 is displayed, as a bore axis target 414, as illustratedon the computer display 410 of FIG. 3. The computer system 400 mayrepresent the bore axis target 414 in any number of ways. For instance,the bore axis target 414 may be visually represented as cross-hairs, ora dot, on a computer display 410. Alternatively, the bore axis target414 may only exist as stored data, such as coordinates, in the memory ofthe computer system 400 or the coordinates may be displayed numerically.While the computer system 400 is generally discussed and illustratedherein as a laptop computer, or traditional central processing unit, thecomputer system 400 may be virtually any processor platform, as onlyminimal computing power is required for this method. For example, thecomputer system 400 may be conveniently packaged in a portableprogrammable logic controller for ease of use on a production line.

The arbor 300 is then removed from the bore 110 and the workpiece 200 isinserted into the bore 100, as illustrated in FIG. 4. The proximal end212 of the workpiece rod 210 generally extends from the proximal end 114of the bore 110, although that is not necessary. The distal end 214 ofthe rod may generally rest on the distal end 116 of the bore 110 duringapplication of this method to this particular embodiment. Next, theposition of the rod 210 in a plane transverse to the central axis CX issensed, preferably in the same manner as the arbor pin position wassensed, and the rod position data is transmitted to the computer system400. Of course it is not necessary to directly sense the rod itself ifanother reference surface is accurately positioned on the rod, such as acoaxial extension of the rod.

The computer system 400 then computes a center of the rod 210 from therod position data and represents the rod center as a rod target 412. Aspreviously discussed with reference to the bore axis target 414, thecomputer system 400 may represent the rod target 412 in any number ofways, such as those described above. For instance, the rod target 412may be visually represented as cross-hairs, or a dot, on a computerdisplay 410. The computer display 410 of FIG. 5 illustrates both thepreviously determined bore axis target 414 and the rod target 412 on thesame computer display 410. The rod target 412 of FIG. 5 is not alignedwith the bore axis target 414 and is therefore not on the central axisCX.

Lastly, the position of the rod 210 is adjusted to bring the rod target412 substantially coincident with the bore axis target 414, indicatingthat the rod 210 is aligned on the central axis CX. This step ofadjusting the rod 210 position may be accomplished manually or it may beautomated. In many applications the degree to which the rod 210 positionneeds to be adjusted is very small and requires finely calibratedequipment.

In embodiments where the bore axis target 414 and the rod target 412 areactually displayed on a computer display 410, as seen in FIG. 5, anoperator may visually identify the location of the rod 210 with respectto the position of the central axis CX, or bore axis target 414, andadjust the location of the rod 210 accordingly until the rod target 412and the bore axis target 414 are substantially coincident.Alternatively, embodiments wherein the rod target 412 and bore axistarget 414 only exist as stored data in the memory of the computersystem 400, the computer software may incorporate and display vectorinstructions to an operator to substantially align the targets 412, 414,or the computer system 400 may instruct an automated positioning systemto substantially align the targets 412, 414. As one with skill in theart will appreciate, the computer system 400 may incorporate audiblesignals to instruct the alignment or indicate how close the rod target412 is to the bore axis target 414.

Following alignment, some type of support or guide structure associatedwith the rod 210 is usually then tightened in position, as describedbelow with respect to the preferred embodiment.

The method and apparatus of the invention have particular application inthe field of free piston machines, particularly free piston Stirlingcycle coolers and engines, where it is sometimes essential to have thecenter of a displacer rod coaxial with the central axis of a cylinder.While the shapes of the cylindrical bore 110 and the workpiece 200 inFIG. 1 through FIG. 5 are similar to those of a free piston, Stirlingcycle machine, application of the current method to a free piston cooler700 is illustrated in detail in FIG. 6 through FIG. 14, and is describedbelow.

With reference now to FIG. 6, FIG. 7A, and FIG. 7B, the free pistoncooler 700 has a cold head 710 and a warm end 720. The free pistoncooler 700 includes a piston 730, driven by a piston driver 732,reciprocating partially within a cylinder 750, and a displacer 740having a displacer rod 742, passing through an inner bore of the piston730, attached to a displacer spring 744 by a connector 746, all enclosedin a housing 770. The piston driver 732 is a linear motor having anarmature winding 733 which drives magnets 734 that are fixed to thepiston 730, as seen in FIG. 7B. The displacer spring 744 that isillustrated is a planar spring like that shown in U.S. Pat. No.5,525,845. The housing 770 is removed in FIG. 7A and FIG. 7B and thecomponents of the free piston cooler 700 that reciprocate are shown inFIG. 7B, while the stationary components are illustrated in FIG. 7A. Thedisplacer 740 reciprocates within the lower portion of the cylinder 750,indicated by motion indicator Md in FIG. 7B, between the cold head 710and the warm end 720, and the piston 730 reciprocates within the upperportion of the cylinder 750 on the displacer rod 742 and bounded by thecylinder 750.

During operation of the free piston cooler 700, the piston driver 732,typically an electric linear motor, moves the piston 730 in thedirections of motion indicated by Mp in FIG. 7B. The movement of thepiston 730 from a first position to a second position, where thedirection of travel reverses, defines a stroke, also referred to asamplitude. In operation, a working fluid, generally helium, istransported, compressed, and expanded by the combined movement of thepiston 730 and the displacer 740. As previously mentioned, the movementof the piston 730 is effected by the piston driver 732.

The motion of the displacer 740 is the result of many combined actionsincluding, but not limited to, changes in the working fluid pressurecreated by the movement of the piston 730, energy storing spring effectsin the free piston cooler 700 introduced by the displacer spring 744,changing properties of the working fluid, and other internal displacercontrol devices. The movement of the displacer 740, indicated by Md,shuttles the working fluid between the cold head 710 and the warm end720, generally through a working fluid passage 760 having a regenerator762, illustrated in FIG. 7A.

The regenerator 762 consists of an energy storage medium to and fromwhich the working fluid may transfer energy as it cycles from the coldhead 710 to the warm end 720, and back again. Modern regenerators 762may incorporate pieces of fine porous metal between the cold head 710and the warm end 720 to prevent unnecessary heat loss and improveefficiency. Heat, indicated by Q, is absorbed at the cold head 710during expansion of the working fluid and heat Q is rejected at the warmend 720 during compression of the working fluid. Heat exchangers aregenerally attached to the cold head 710 and the warm end 720 to improvethe transfer of thermal energy to, and away from, the free piston cooler700.

The piston 730 in the free piston cooler 700 reciprocates within and isbounded by the cylinder 750. The piston 730 is suspended away from thecylinder walls with a gas bearing system. Peak efficiency is achieved inpart by ensuring an extremely close fit, such as 12—13 microns radialclearance between the piston 730 and the cylinder 750. The displacer rod742 reciprocates within and is bounded by the inner bore of the piston730. The reciprocating motion between the piston 730 and the displacerrod 742 is out of phase and of different stroke lengths, resulting in arelative motion between the two parts. There is no gas bearing systemthat will suspend the displacer rod 742 away from the inner bore of thepiston 730, therefore, near perfect alignment between the center of thecylinder 750 and the displacer rod 742 is desired so that the displacerrod does not contact the inner bore of the piston 730. Contact betweenthe displacer rod 742 and the inner bore of the piston 730 will causewear on the displacer rod 742 surface and severe contact pressure canovercome the piston 730 gas bearing causing the piston 730 surface tocontact the cylinder 730 surface which can cause undesired wear andpossible free piston cooler 700 failure. Therefore, one application ofthe present invention is to align the displacer rod 742 of a free pistoncooler 700 with the central axis CX of the cylinder to avoid wear of theinterfacing, sliding surfaces.

Application of the present method to the free piston cooler 700 beginswith the insertion of the arbor 300 into the cylinder 750, asillustrated in FIG. 8 and FIG. 12. Next, at least one light beam LB isgenerated and transmitted, by the optical location determining system510, across the central axis CX. Generally, the casing 780 is formedwith holes through which the light beam LB can pass. Further, theoptical location determining system 510 is configured so as toconveniently clamp onto the casing 780 with at least one clamping device800, shown in FIG. 12. This is better illustrated in the top plan viewof FIG. 9. In the embodiment of FIG. 12, four clamping devices 800secure the optical location determining system 510 to the casing 780 andtake the form of large knurled screws 800.

Two light transmitters 520, 530 generate and transmit beams of light524, 534 across the reference pin 320 of the arbor 300. As the referencepin 320 interferes with the first light beam 524 a first light beamshadow 526 is created and cast upon the first light receiver 522.Similarly, as the reference pin 320 interferes with the second lightbeam 534 a second light beam shadow 536 is created and cast upon thesecond light receiver 532. One exemplary embodiment incorporates anoff-the-shelf optical location determining system 510, namely a Keyenceoptical positioning system. In this embodiment, the light transmitters520, 530 are Keyence LS 7030T transmitters and the light receivers 522,532 are Keyence LS 7030R receivers.

In this particular embodiment the first light beam 524 and the secondlight beam 534 are orthogonal to each other, however this is notrequired. Virtually any predetermined relationship between the lightbeams 524, 534 may work with this method. Similarly, while is it mostconvenient to utilize a symmetrical reference pin 320, it is not arequirement provided that the position of the reference pin 320 and thelight beams 524, 534 are carefully positioned to a predeterminedrelationship. Any asymmetry needs to be accounted for in the software,specifically in the mathematical computations made by the computer.Likewise, one with skill in the art will appreciate that the presentmethod will work equally as well with workpiece rods 210 that arenoncircular, or even asymmetric, provided predetermined relationshipsare established and accounted for in the software.

The light receivers 522, 532 then transmit pin position data, determinedfrom the locations of the shadows 526, 536, to the computer system 400.The computer system 400 then computes the center of the reference pin320 and represents the center as the bore axis target 414, seenpreviously in FIG. 3, which is also the location of the central axis CX.

Next, the arbor 300 is removed from the cylinder 750 and thereciprocating components, namely the displacer 740, the displacer rod742, the piston 730, and the displacer spring 744 are positioned asillustrated in FIG. 10, FIG. 13, and FIG. 14. Conveniently, the gasbearing system of the displacer 740 effectively centers the lowerportion of the displacer rod 742 in the cylinder 750 during cooleroperation, thereby only requiring accurate positioning of the end of thedisplacer rod 742 nearest the displacer spring 744. Again, at least onelight beam LB is generated and transmitted across the central axis CX.This is better illustrated in the top plan view of FIG. 11. Here, thetwo light transmitters 520, 530 now generate and transmit beams of light524, 534 across the displacer rod 742. As displacer rod 742 interfereswith the first light beam 524 a first light beam shadow 526 is createdand cast upon the first light receiver 522. Similarly, as the displacerrod 742 interferes with the second light beam 534 a second light beamshadow 536 is created and cast upon the second light receiver 532. Sincethe edges of the shadow are detected by light receivers, the position ofthe center of the each shadow can be computed to providetwo-dimensional, numerical coordinate position data in both the x and yaxes. These numerical coordinates represent the position of the centerline of the cylinder so long as the optical system is not moved duringthe subsequent process of removing the arbor and inserting the displacerand piston.

The light receivers 522, 532 transmit rod position data, determined fromthe locations of the shadows 526, 536, to the computer system 400. Thecomputer system 400 then computes the center of the displacer rod 742and represents the center as the rod target 412, as previously seen inFIG. 5. As previously disclosed, the displacer rod 742 may now bemanually, or automatically, repositioned until the rod target 412 issubstantially coincident with the bore axis target 414. The embodimentof FIG. 12, FIG. 13, and FIG. 14 illustrates a plurality of adjustmentdevices 810, namely precision calipers, used to position the displacerspring 744 so that the displacer rod 742 is substantially centeredwithin the cylinder 750. Once the proper position is located, thedisplacer spring 744 is attached to the casing 780 to fix the locationof the displacer rod 742.

It is important that the location determining system be rigidly fixed tothe body in which the cylindrical bore is formed. Any movement of thelocation determining system with respect to the body during performanceof the alignment will generate erroneous position data. Therefore, it isdesirable to incorporate a sensing system to detect such motion, inputits data to the computer and that the computer software be written tomonitor the sensing system and display or sound a warning that movementhas occurred and the alignment results are erroneous. The preferred wayof accomplishing this is to sense the position of the four holes throughthe casing through which the light beams are transmitted. These holesare arranged in two orthogonally aligned opposite pairs of holes throughthe casing. Two such holes 782 and 784 are visible in FIGS. 12 and 13.Light transmitters are used which transmit a light beam that is widerthan the holes through the casing so that the light falling upon thelight receivers has outer boundaries representing the shadow cast by thecasing at the edges of the holes in the outer casing and an inner shadowrepresenting the arbor pin or the displacer rod. This creates two spacedlight beams falling upon the light receivers. The innermost boundariesof those light beams represent the outer edges of the arbor pin or thedisplacer rod and the outermost boundaries represent the edges of theholes. These edges provide data representing the location of the holeedges and that data is also transmitted to the computer. The computerthen monitors the position of those hole edges and, if they move duringthe alignment process, the computer then signals that such motion hasoccurred and therefore the alignment may be in error.

While the description of the free piston cooler embodiment is directedtoward an optical location determining system, one with skill in the artwill appreciate that the previously disclosed electromechanicaldetermining system may be just as easily applied.

Numerous alterations, modifications, and variations of the preferredembodiments disclosed herein will be apparent to those skilled in theart and they are all anticipated and contemplated to be within thespirit and scope of the instant invention. For example, althoughspecific embodiments have been described in detail, those with skill inthe art will understand that the preceding embodiments and variationscan be modified to incorporate various types of substitute and oradditional or alternative materials, relative arrangement of elements,and dimensional configurations. Accordingly, even though only fewvariations of the present invention are described herein, it is to beunderstood that the practice of such additional modifications andvariations and the equivalents thereof, are within the spirit and scopeof the invention as defined in the following claims.

1. A method for centering a workpiece on the central axis of acylindrical bore in a body, the workpiece including a rod, the methodcomprising: (a) inserting into an end of the bore an arbor having acylindrical exterior surface matingly slidable within the cylindricalbore, the arbor having a symmetrical reference pin protruding from anend of the arbor coaxially with the central axis; (b) sensing theposition of the reference pin in a plane transverse to the central axisand transmitting pin position data to a computer system; (c) computing acenter of the reference pin from the pin position data and representingthe reference pin center as a bore axis target; (d) removing the arborfrom the bore and inserting the workpiece into the bore with the rodprotruding from the end of the bore; (e) sensing the position of the rodin the plane traverse to the central axis and transmitting rod positiondata to the computer system; (f) computing a center of the rod from therod position data and representing the rod center as a rod target; and(g) adjusting the position of the rod to bring the rod targetsubstantially coincident with the bore axis target.
 2. The method inaccordance with claim 1, wherein the position of the reference pin andthe position of the rod are electromechanically sensed.
 3. The method inaccordance with claim 1, wherein the position of the reference pin andthe position of the rod are optically sensed.
 4. The method inaccordance with claim 3, wherein the position sensing comprisesgenerating and transmitting a beam of light from each of a plurality oflight transmitters across the central axis to cooperating lightreceivers, the reference pin and rod casting a shadow upon the opposingreceivers to provide an indication of the location of the reference pinor the rod with respect to the central axis.
 5. The method of claim 4,wherein two light beams are transmitted orthogonally across the centralaxis between two cooperating pairs of light transmitters and lightreceivers.
 6. The method of claim 4, wherein the light transmittersincorporate light emitting diodes to generate the beams of light and thelight receivers are charge coupled devices.
 7. The method in accordancewith claim 1, wherein the bore is a bore of a free piston machine, theworkpiece is a connecting rod of a free piston machine, and a displaceris positioned at a distal end of the connecting rod and substantiallycenters the distal end of the connecting rod within the bore.